Demon D3O Rocks
I use both the knee and the elbow pads from Demon for DH mountain biking. They're extremely comfortable and I would argue they're among the most comfortable in the industry. But, what sets them apart from the rest is that they use a superior padding to closed cell foam...D3O. This stuff is amazing it hardens up on impact and offers unparalleled protection. I've taken many diggers on the bike and they perform great. They stay in place when I go down and they're flexible when I'm pedaling to the point that I don't even know they're there.